Animating a circular UIBezierPath
Using CAShapeLayer is much easier and cleaner. The reason is that CAShapeLayer includes properties strokeStart and strokeEnd. These values range from 0 (the beginning of the path) to 1 (the end of the path) and are animatable.
By changing them you can easily draw any arc of your circle (or any part of an arbitrary path, for that matter.) The properties are animatable, so you can create an animation of a growing/shrinking pie slice or section of a ring shape. It's much easier and more performant than implementing code in drawRect.